1. Field of the Invention
The present invention is directed to apparatus and methods for oxidizing fuel in combustion systems. More particularly, in one embodiment the present invention is directed to methods and apparatus for oxidizing fuel in order to control puffing by incinerators of generally high toxic unoxidized and partially oxidized organic vapors to the environment. In another embodiment the present invention is directed toward the combustion of fuel with very low emissions of NO.sub.x.
2. Background Art
The United States currently produces 265 million tons per year of hazardous waste. In most instances the toxicity of this waste comes from toxic organic materials which, in principle, can be completely destroyed by incineration. In contrast to incineration, all other disposal technologies involve the risk that some of the toxic materials will return to the environment. Thus, incineration, in principle, is the ideal solution to the problem of waste disposal.
Available incinerator technology, however, is subject to a number of limitations. One of the most important of these limitations is the occasional emission of puffs of toxic organic materials into the environment by rotary kiln incinerators. This problem, referred to as "puffing," is serious within the incineration field, since rotary kiln incinerators are a substantial fraction of total U.S. incineration capacity.
The severity of an incinerator's puffing problems depends upon a number of parameters: the rate of radiative heat transfer, the rate at which fresh surface is exposed, the frequency at which slugs of material capable of forming puffs are added to the incinerator, and the size of those slugs. The latter two parameters are directly controllable while the former two are partially controllable by changing operating parameters such as the rate of rotation. Thus, while it is possible for an incinerator to manage puffing problems, this means operating the incinerator at a capacity that is less than the capacity it would have if puffing were not an issue.
Rotary kiln incinerators handle both solid and liquid wastes. For combustible liquid wastes the practice is to mix the liquid waste with a sorbent, which is then placed in a container (typically a cardboard, plastic, or steel drum), and fed to the rotary kiln incinerator. These large closed containers are heated until the vapor pressure of the liquid is sufficient to cause them to rupture. This results in a sudden discharge of a large amount of combustible vapors into the incinerator.
Often, when this sudden discharge occurs, the supply of combustion air within the incinerator can be much less than sufficient for complete oxidation of these suddenly released vapors. This can cause substantial amounts of these toxic organic vapors to be discharged from the incinerator into the environment without first being oxidized.
A problem very similar to puffing is likely to occur during the U.S. Army's planned disposal of its chemical weapons.
Generally, the Army's plan of disposal of nerve agent mines and other chemical agent munitions consisted of bringing munitions into a negative pressure building and unpacked, the packing material going to a dunnage incinerator. Some of these munitions (i.e., the bombs and ton containers) do not contain energetic materials. The nerve agents are drained from these munitions and sent to a liquid incinerator, and the empty munitions go to another incinerator, the metal parts furnace. Other munitions, such as rockets, mines and shells, do contain energetic materials. These energetic materials are decades old and in some instances their stability is questionable. Nevertheless, these old explosives must be handled, taken out of the rocket, mine, or projectile, and sent to another incinerator, the "deactivation furnace system." Following this, rockets, mines and projectiles are drained of their chemical agent, with the agent again going to the liquid incinerator and the empty munitions going to the metal parts furnace.
The incineration systems the U.S. Army presently plan to build involves unpacking these overage munitions, removing whatever explosive materials they contain, and draining them of nerve agent. The safety problems involved in doing this much handling are large and expensive to solve. Consequently, the disposal of these munitions is expected to cost billions.
GA Technologies has done an analysis of the risks involved in the disposal of these weapons. In this analysis, the greatest risk in disposing of the weapons, was found to be the accidental feeding of a mine or other munition, which had not been emptied of its nerve agent into the packing material incinerator. The nerve agent contained within the mine would suddenly be vaporized within the incinerator, thereby causing a nearly instantaneous release of combustible organic vapor into the incinerator. The supply of combustion air would not be nearly adequate for complete combustion of this nerve agent, and so the nerve agent would be released in an unoxidized state. Up to 15 pounds of nerve agent could be released to the atmosphere in a puff.
GA Technologies estimate the probability for this accident at 0.01 per year per site, and the U.S. Army's present plans call for building nine facilities of this general design. The Army regards the risk of discharging 15 pounds of nerve gas to the environment as marginally acceptable since the probability of the event is not high and the amount of nerve gas released small enough that casualties in the downwind civilian population are unlikely. However, since there are nine sites planned, and the destruction of the munitions will require a number of years, the probability of such an accident happening at least once is significant.
Generally, incinerators are evaluated in terms of the fraction of the input organic which they destroy, i.e., the DRE or the destruction and removal effectiveness. Despite their tendency to occasionally puff, most rotary kiln incinerators achieve a sufficiently high DRE on average so that their operations are at least marginally acceptable from an environmental viewpoint.
Regulations regarding the operation of incinerators are written in terms of the destruction and removal efficiency (the DRE), and typically their requirement for the DRE average is 99.99%. In cases of incinerators which handle extremely toxic materials, a DRE of 99.9999% is sometimes required. This requirement, however, relates to the DRE measured as a rolling average over a period of time. Thus, a technology which eliminates puffs need not itself be 99.99% effective in order to be satisfactory. It is sufficient that the incinerator using this technology have a DRE of 99.99% or 99.9999% on a time average basis. Hence, in most situations in which puffing is a problem, a technology which decreases the size of the puff being emitted to the atmosphere by a factor of 10 to 100 would be satisfactory.
However, the technology will be useful only if it has sufficient capacity. For the sake of a numerical example, a puff control device with a residence time of 0.5 seconds at a temperature of 800.degree. C. should be considered, this device treating a puff of toluene having a median height of 5,000 ppm and a duration of 34 seconds. Oxidizing one mole of toluene requires 11 moles of oxygen or 55 moles of air. Consequently to completely oxidize the puff in this example the device would need a capacity equivalent to 18.7 cc of air per cc of the devices' volume.
As another example, 15 pounds of the nerve gas GB (C.sub.4 H.sub.10 FO.sub.2 P) is suddenly introduced to a 5 megawatt incinerator using combustion air at 4.66 pounds per second. This corresponds to 0.16 pound moles of air per second and 0.107 pound moles of GB. Oxidation of 0.107 pound moles of GB will require 3.48 pound moles of air. Since the gas passing through the puffing control device has a residence time of 0.5 seconds, the device has a volume sufficient to hold 0.08 pound moles of air. Thus, to completely oxidize the nerve gas the device would need a capacity equivalent to 3.48/0.08 or 43.5 cc of air per cc of the device's volume.